Hydro-Mechanical Response of Wildfire-Burned Soils under Varying Burn Severity: Implications for Post-Wildfire Slope Stability

Wildfires can substantially alter the hydro-mechanical behavior of near-surface soils, increasing slope vulnerability to rainfall-induced instability. This study examines how wildfire-simulated thermal exposure modifies the hydraulic, mechanical, and microstructural characteristics of mulch-amended soils and the resulting implications for slope stability. Laboratory experiments were performed on unburned soils and soils subjected to controlled heating representing low (150°C), moderate (300°C), and high (600°C) burn severities. Testing included grain size distribution, Atterberg limits, compaction, soil–water characteristic curves (SWCC), unsaturated hydraulic conductivity, thermal conductivity, swelling behavior, and saturated direct shear strength. Microstructural and mineralogical changes were analyzed using field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). Coupled seepage–stability analyses were conducted to evaluate slope response during an extreme rainfall event (116.3 mm in 24 h). Thermal exposure progressively transformed the soil fabric from a fines-supported matrix to a sand-dominated granular structure, accompanied by reduced plasticity, diminished swelling potential, and lower saturated volumetric water content. SWCC results showed systematic shifts with burn severity, including reduced air-entry value and moisture retention capacity at high burn severity, indicating altered pore-size distribution and a diminished ability to sustain matric suction during infiltration. Hydraulic conductivity increased by four orders of magnitude from unburned soil to high burn severity soil, reflecting progressive modification of pore connectivity and flow pathways following thermal alteration of soil fabric and organic matter. Thermal conductivity increased with moisture content for all soils, with the largest increase observed at moderate burn severity (300°C), likely reflecting improved particle contacts and pore connectivity. Direct shear tests indicated decreasing cohesion but increasing friction angle with burn severity due to ash and biochar residues. Microstructural analyses revealed kaolinite dehydroxylation to metakaolin and the formation of a thermally induced surface crust at 600°C. Slope stability simulations showed that the unburned, low-burn-severity, and moderate-burn-severity soils maintained factors of safety above 1.8 under the simulated rainfall event, whereas the high-burn-severity condition developed a shallow surficial failure. These results provide a plausible mechanistic basis for understanding how burn severity governs the hydro-mechanical evolution of soils and the susceptibility of post-wildfire slopes to rainfall-triggered instability.